This is a reeeeeeally common thing. ESPECIALLY when they're little. He's getting almost no nurtrition from the tablefood... so his body is craving what's easily digestable and what's easy to eat. Expect it to last a few weeks.
Even adults though, when they're sick don't want to eat or want "comfort food". But we ALL crave easily digestible protein & fat rich foods. (Our immune system is predominantly made up of fats/ as are antibodies with just a little protein thrown in for structure. Hence why chicken soup is an illness food. And why we'll have/scarf McDonalds when sick even when nothing else sounds good... fast food is loaded in fats and is almost predigested for us).
Fruits and veggies have none of the nurtients our body needs to fight the infection... but milk has all of them (including the vitamins and minerals that aid it).
You honestly don't need to worry about food intake when a person is ill hardly at all. HYDRATION, yes. Food.. not so much. Our digestive system almost shuts down entirely (it uses a ton of energy that gets shunted to our immune system). So you DO want to give them high fat foods whenever they can be coaxed to eat... but it works out perfectly with babes... since the milk has both the fluid and nutrient content.